1. Field of the Invention
The present invention relates to a sensor for detecting combustible gas, particularly carbon monoxide contained in the general atmosphere or exhaust gas of various combustion equipment using gas and petroleum for fuel, and more particularly to a gas sensor provided with excellent characteristics with respect of stability of sensor operation under severe working environment as well as with respect of durability which raises the most serious problem in chemical sensors. However, this invention is applicable to a wide variety of chemical sensor system gas sensors used for various objects with respect to various gaseous components.
2. Description of the Prior Art
Carbon monoxide is a gas which has no color, no taste, and no scent, and is slightly lighter than air but highly toxic, and even at a concentration as low as 200 ppm, breathing the gas for 2 to 3 hours causes headaches, and at a concentration higher than 3000 ppm, death results by breathing for about 10 minutes and at 6000 ppm or higher, for a few minutes.
Because even in general home, carbon monoxide is generated by an instantaneous water heater, bath boiler, petroleum space heater, gas space heater, or charcoal fire, a carbon monoxide gas detecting sensor which can be contained in these apparatus or installed indoors, and is inexpensive, small-size, and highly reliable is strongly desired.
Examples of a gas sensor, particularly chemical sensor for detecting carbon monoxide which has been proposed to date include a type in which an electrode is mounted for absorbing and oxidizing carbon monoxide in an electrolyte and detects the carbon monoxide concentration from a current value in proportion to the carbon monoxide concentration (controlled potential electrolysis type gas sensor), in which the gas sensor is further divided into two types; one is a coulometric type in which a generated current output is to be measured and the other is a potentiometric type in which a generated voltage output is to be measured. Therefore, the coulometric type can not avoid electrolytic reaction on the electrode surfaces, resulting in deterioration of the electrode leading to an effective life shorter than that of the potentiometric type;
a type for detecting gas using a sintered compact type of an N-type semiconductor oxide, for example, stannic oxide sensitized by adding traces of metallic element such as noble metals and utilizing characteristics of these semiconductors which vary electric conductivity when in contact with combustible gas (semiconductor type gas sensor);
and a type for attaching alumina to about 20-.mu.m platinum thin wire and heating it to a constant temperature by using a pair of reference elements with and without bearing noble metal and detecting a difference of heat generation when combustible gas comes in contact with this element to carry out catalytic oxidation reactions (contact combustion type gas sensor).
For example, there is a detailed description in "Reference 1," Chapter 14, Basics of Gas Sensors (written by Masaki Haruda) in "Sensor Practical Dictionary" in P. 112-130 supervised by Toyoaki Ohmori and published by Fuji Techno System (1986).
There also proposed a solid electrolyte type carbon monoxide sensor for detecting carbon monoxide by constituting a zirconia electrochemical cell and forming platinum/alumina catalyst layeron one side of the electrode [for example, see H. Okamoto, H. Obayashi, and T. Kudo; Solid State lonics, 1, 319 (1980)].
The principle of this solid electrolysis type carbon monoxide sensor relies on a kind of oxygen concentration cell formed on electrodes on the catalyst layer side and naked side, in that it utilizes that at the electrode on the catalyst side, oxygen directly reaches the electrode as it is and carbon monoxide does not reach, whereas on the naked-side electrode, both oxygen and carbon monoxide reach and this carbon monoxide reduces oxygen and forms an oxygen concentration cell across both electrodes, and the electromotive force output appears.
As semiconductor type carbon monoxide sensors, in "Reference 2," Japanese Patent Publication No. Sho 53-43320 and in "Reference 3," Japanese Patent Application Laid Open No. Sho 61-50051, there proposed were methods and their improvements for detecting carbon monoxide gas by heating a gas sensor utilizing changes of resistance of a metallic oxide semiconductor in the high-temperature and low-temperature regions alternately and sampling the gas sensor output in the low-temperature region intermittently. These are characterized by the point in that the selectivity of carbon monoxide detection is improved primarily by contrivances from the viewpoint of signal processing.
In "Reference 4," Japanese Patent Application Laid Open No. Hei 1-227951, there proposed were gas sensors using as a sensor proper a metallic oxide with the resistance varied in accord with gases, in which a zeolite covered layer was provided on the surface of the sensor proper. This also aims at improving the selectivity of carbon monoxide detection.
With respect to the gas selective permeation element, ceramic gas separating membrane, that is, inorganic separating membrane has been proposed [for example, see Tatsuya Okubo and Seiji Morooka, "Current Status of Inorganic Separating Membrane and Future Development," Chemical Engineering, 12, 1 (1988, 1989)], but any proposal for applying the inorganic separating membrane to gas sensors has not yet been made to date.
All these chemical sensors have following defects. That is, controlled potential electrolysis gas sensors, semiconductor type gas sensors, and contact combustion type gas sensors basically provide characteristics for detecting hydrogen, alcohol, etc. other than carbon monoxide (CO), even if various contrivances are, in principle, made for indiscriminately reacting with reducing gas (combustible gas). That is, they have a defect in that the selectivity of CO is poor. They also have defects in that the sensor and the sensor system are, in general, expensive and the signal processing circuit of the sensor becomes complicated. Except for the contact combustion type, they have a defect of poor controllability because the sensor output to the CO concentration is nonlinear.
In particular, the biggest problem of chemical sensors which have been extensively used as gas sensors is that they cannot help becoming a fail-out detection system in spite of being a decisive sensor which involves in the safety. This is attributed to the principle in that the signal as a sensor becomes zero when no carbon monoxide is detected, and a signal is outputted when carbon monoxide is detected, and this output signal lowers as the sensor deteriorates.
To specifically describe the fail-out problem, for example, assume that using a carbon monoxide sensor, the equipment is designed under conditions to set a boundary value for the concentration of carbon monoxide and to stop the equipment when the carbon monoxide concentration exceeds this boundary value because trouble occurs from the viewpoint of safety. It is a fail-safe design philosophy to design the equipment to operate on the safety side as an equipment even if any trouble should occur, but in the case of a conventional chemical sensor system carbon monoxide sensor, there is a danger in that the sensor does not operate due to trouble caused by deterioration even though carbon monoxide is, in actuality, generated more than a certain boundary value. This is because the system is not designed to be fail-safe but fail-out, constituting a fatal problem from the viewpoint of system safety. This relates to the fact that even if a problem of disconnection of a heating means is able to be detected with respect to sensor trouble, whether the sensor itself is deteriorated or not is unable to be determined. This also relates to the shorter sensor life as compared to the equipment life.
The case in which danger of imperfect combustion increases when a gas sensor is mounted to combustion equipment for detecting imperfect combustion is more likely to occur when the combustion equipment has been used over a considerably long time, but in such event, there is a danger in which deterioration of the gas sensor has progressed, creating a problem that imperfect combustion is unable to be detected when the output signal lowers due to deterioration of the gas sensor.
Lowering, that is, deterioration, of the chemical sensor output is attributed to deterioration of electrodes or catalysts which play leading roles in the chemical sensor with time as reactions take place, and this deterioration is attributed to detection reactions of carbon monoxide being inhibited by the catalyst reduced by reducing gas such as hydrogen or hydrocarbons existing in the combustion exhaust gas or sulfur-based compounds strongly adsorbed onto the electrode surface. In these chemical sensors, noble metals are frequently used for the electrode or catalyst which plays an important role in sensor capabilities, but these noble metals are susceptible to sulfur-based compounds or silicon-based compounds and are easy to deteriorate, and has a problem of being extremely difficult to secure durability. Because the hydrocarbons coexisting in the exhaust gas of combustion equipment have large molecular weight and large molecular size, once they are adsorbed on the surface of noble metals such as platinum, etc., there are problems in that they inhibit adsorption of carbon monoxide and exert adverse effect as disturbing gas.
In addition, since the sensor system is, in principle, not designed to work on the fail safe side, a sensor with extremely high reliability in durability is required in order to put this into practical use with high reliability, but presently, there is a problem that no sensor system with a precisely established warranty in durability has been realized in the ideological level.